The present invention relates to the detection of objects in a body of water, and, more particularly to a bottom moored and tethered underwater sensor system deployable in a wide range of water depths.
Many military missions involve surveillance of bodies of water to support submarine detection. Sonobuoys are often deployed to aid in the surveillance operations. Passive sonobuoys sense sounds, such as those sounds produced by propellers and machinery. Active sonobuoys bounce a sonar signal off the surface of an object such as a submarine. Active sonobuoys generally require a greater operating depth than passive sonobuoys.
Sonobuoys typically include a buoyant chamber containing one or more sensors and a transmitter. Sonobuoys can be dropped from aircraft equipped with a means to launch the sonobuoys and electric equipment to receive and process data from the sensors. Sonobuoys can also be hand launched over the side of a ship. In some instances, the sonobuoys include a seawater battery wich is energized upon salt water contact, which in turn activates a mechanism for inflating a urethane material float with CO2 gas, which starts an ocean deployment sequence. Sonobuoys typically contain one or more sensors, such as hydrophones, for converting underwater sound to electrical signals which are amplified and transmitted via radio frequency (RF) to a surveillance receiver (eg., a P-3 maritime patrol aircraft).
During submarine detection activities in an ocean, a maritime patrol aircraft is normally directed to a surveillance area by other surveillance assets. The aircraft typically deploys multiple sonobuoys over a substantial area while attempting detection of the submarine. Upon detection of the submarine, the maritime patrol aircraft follows the submarine as it traverses an area of the ocean. Many sonobuoys are deployed in such a mission. A typical sonobuoy employed in such an operation has a life of approximately 8 hours, and automatically floods and sinks to the ocean bottom upon reaching its maximum service life.
Other surveillance missions, such as harbor defense and force protection use underwater acoustic sensors deployed in shallow water. Shallow water is nominally defined as a depth of approximately 4 feet to approximately 1000 feet. The targets of interest for these shallow water surveillance missions include: submarines, mini-submarines and high-speed, shallow-draft, surface craft. Shallow water surveillance missions typically employ fixed sensors for surveillance activities.
It is known in the art to employ bottom moored, omnidirectional sensors (i.e., scalar sensors) for surveillance missions. Omnidirectional sensors receive and convert the sound pressure wave energy generated by an underwater target object to a voltage. Omnidirectional sensors are capable of measuring only signal magnitude.
Directional frequency analysis and ranging (DIFAR) sensors have become increasingly important in sonobuoy applications. Unlike previous sensors, such as omnidirectional sensors that measure only signal magnitude, DIFAR sensors provide a magnetic bearing to contacts of interest. This added feature allows trackers to track a contact of interest with as few as one DIFAR sonobuoy, and fix the location of the contact with as few as two DIFAR sensors.
FIG. 1 illustrates a passive DIFAR sonobuoy 30 utilized by the United States Navy, and specifically referenced by number (AN/SSQ-53A). Sonobuoy 30 includes a surface float 32 having positive buoyancy deployed at an upper surface 34 of a body of water 31 to be monitored. Surface float 32 includes an antenna for transmitting detection data from sonobuoy 30 to a remote receiving station (not shown). Since DIFAR sonobuoys are extremely sensitive to the effect of flow across the sensor, sonobuoy 30 also includes a compliant assembly 36 suspended from top float 32 for providing and maintaining proper positioning for subsurface electronics 38 and hydrophones 40. Sonobuoy 30 further includes a drogue (i.e., sea anchor) 42 and a mass/damper 44 for limiting hydrophone 40 movement. In a typical deployment, the distance between surface float 32 and hydrophone 40 is, at a minimum, approximately 90 feet, and at a maximum, a proximately 1000 feet.
In view of the above, there is a need for a single type underwater sensor system which is capable of operating covertly in a wide variety of water depths, including very shallow water. This sensor system preferably employs a directional sensor, such as a DIFAR sensor, so that bearing information may be obtained on detected objects. The underwater sensor system must be deployed (i.e., oriented) in a stable configuration so that the directional sensors provide accurate measurements.
The present invention provides an apparatus including a mooring platform for anchoring the apparatus to a bottom ground surface covered by a body of water. The apparatus also includes a buoyant chamber, and a directional frequency analysis and ranging (DIFAR) sensor positioned within the buoyant chamber. The DIFAR sensor includes an x hydrophone pair for sensing sounds along an x axis, and a y hydrophone pair positioned orthogonally to the x hydrophone pair for sensing sounds along a y axis. Finally, the apparatus includes a tether having a first end and a second end. The first end of the tether is coupled to the mooring platform and the second end of the tether is coupled to the buoyant chamber to suspend the buoyant chamber a pre-defined distance from the bottom ground surface. The tether also orients the x hydrophone pair and the y hydrophone pair so that the x axis and the y axis form a plane substantially co-planer to a top surface of the body of water.
The buoyant chamber of the present invention can exhibit either positive or negative buoyancy. In the case of negative buoyancy, the mooring platform includes a raised arm for suspending the buoyant chamber. The apparatus may be powered by either an internal power supply or an external power supply. The external power supply is coupled to the apparatus via a signal cable routed along the bottom ground surface of the water. In one embodiment of the present invention, the buoyant chamber of the apparatus is deployed at a depth from approximately 4 feet to approximately 1000 feet from the top surface of the body of water. The apparatus of the present invention may also be recoverable for later deployment.
The present invention also provides an acoustic surveillance system for detecting objects in a body of water having a top surface, where the body of water covers a bottom ground surface. The acoustic surveillance system includes a mooring platform for anchoring the acoustic surveillance system to the bottom ground surface. The acoustic surveillance system also includes a buoyant chamber, and a compass positioned within the buoyant chamber for providing a directional magnetic reference signal. The acoustic surveillance system also includes a directional sensor positioned within the buoyant chamber. The directional sensor has an x hydrophone pair for sensing sounds along an x axis relative to the directional magnetic reference signal. The directional sensor also has a y hydrophone pair positioned orthogonally to the x hydrophone pair for sensing sounds along a y axis. The acoustic surveillance system further includes a tether having a first end and a second end. The first end of the tether is coupled to the mooring platform. The second end of the tether is coupled to the buoyant chamber to suspend the buoyant chamber a pre-defined distance from the bottom ground surface, and to orient the x hydrophone pair and the y hydrophone pair so that the x axis and the y axis form a plane substantially co-planer to the top surface of the body of water. Finally, the acoustic surveillance system includes a transmitter for transmitting the information detected by the directional sensor to a receiver.
In one embodiment of the present invention, the directional sensor is a directional frequency analysis and ranging (DIFAR) sensor. In alternate embodiments of the present invention, the transmitter transmits the information detected by the directional sensor to the receiver via ether a signal cable routed along the bottom ground surface of the water or a radio frequency (RF) transmission. In the radio frequency transmission embodiment, the transmitter further includes a frequency modulator for modulating the information detected by the directional sensor prior to transmission.
In a preferred embodiment of the present invention, the compass is a magnetic flux compass. In alternative embodiments of the present invention, the hydrophones are either pressure gradient hydrophones or accelerometer hydrophones.
The present invention also provides an acoustic surveillance system for detecting objects in a body of water having a top surface, where the body of water covers a bottom ground surface. The acoustic surveillance system includes mooring means for mooring the acoustic surveillance system to the bottom ground surface. The acoustic surveillance system also includes a buoyant chamber, and a compass positioned within the buoyant chamber for providing a directional magnetic reference signal. The acoustic surveillance system also includes a directional sensor positioned within the buoyant chamber. The directional sensor includes an x hydrophone pair for sensing sounds along an x axis relative to the directional magnetic reference signal. The directional sensor also includes a y hydrophone pair positioned orthogonally to the x hydrophone pair for sensing sounds along a y axis. The acoustic surveillance system further includes tethering means to tether the buoyant chamber to the mooring platform. The buoyant chamber is suspended a pre-defined distance from the bottom ground surface and is oriented so that the x axis of the x hydrophone pair and the y axis of the y hydrophone pair form a plane substantially co-planer to the top surface of the body of water. Finally the acoustic surveillance system includes transmitting means for transmitting the information detected by the directional sensor to a receiver.
In one embodiment of the present invention, the directional sensor is a directional frequency analysis and ranging (DIFAR) sensor. The buoyant chamber of the present invention can exhibit either positive or negative buoyancy.
The present invention also provides a method for deploying an acoustic surveillance system for detecting objects in a body of water having a top surface, where the body of water covers a bottom ground surface. The method begins by anchoring a mooring platform of the acoustic surveillance system to the bottom ground surface of the body of water. Next, a directional sensor assembly having two orthogonal hydrophone pairs and a compass are tethered to the mooring platform such that the two orthogonal hydrophone pairs are oriented in a plane substantially co-planer to the top surface of the water. The two orthogonal hydrophone pairs are further oriented relative to a directional magnetic reference signal provided by the compass. The directional sensor assembly is suspended a pre-defined distance from the bottom ground surface of the body of water.
The directional sensor assembly of this method may include a directional frequency analysis and ranging (DIFAR) sensor. The directional sensor assembly may also include a buoyant chamber, where the buoyant chamber may exhibit either positive or negative buoyancy. The mooring platform includes a raised arm for suspending the buoyant chamber when the buoyant chamber exhibits negative buoyancy.
The present invention offers the advantage of a low cost, reusable underwater acoustic surveillance system which is deployable in a wide range of water depths, including very shallow water. The present invention also offers the advantage of a directional sensor system which enables a user to obtain bearing information for a detected object. Finally, the present invention offers the advantage of a long operating life, since the present invention may be powered by an external power supply via a power/signal cable.